Photosensors are commonly used in a mail-related machine to make sure all parts in the machine function in a coordinated way. For example, in a mail stacker, photosensors can be used to monitor the arrival of a mailpiece so that a certain roller can start to move in order to bring that mailpiece into a mail stack. Photosensors can also be used in the same machine to monitor the size of the growing mail stack so as to notify an operator when to take the mail stack off the mail stacker. In a mail inserting machine where a plurality of enclosure feeders are used to release documents onto the machine chassis and the released documents are pushed into a stack to be inserted into an envelope, photosensors can be used to check the arrival of the envelope, the movement of the released documents and so forth.
A photosensor, in general, comprises a photo-detector and a light-emitting diode (LED). When the photosensor is active, the LED is activated to produce light to illuminate the photo-detector, as illustrated in FIG. 1a. When the photo-detector is not blocked, the illumination causes the photo-detector to become saturated. In this state, the output voltage of the photo-detector circuit is generally low. But when the photo-detector is blocked by an object coming into the space between the LED and the photo-detector as illustrated in FIG. 1b, the photo-detector is in a high resistive state and the output voltage of the photo-detector circuit is generally high or substantially equal to the supply voltage. The photsensors are used not only to detect the incoming mail pieces, but also pieces leaving the sensor.
A plot of the output voltage of a typical photo-detector versus the supply current to the LED (V-I Curve) is shown in FIG. 2. As shown in FIG. 2, the photo-detector becomes saturated at iLED=i_thresh. In general, the output voltage of a photo-detector depends on the light output of the LED, the distance of the photo-detector from the LED, the alignment between the photo-detector and the LED, and so forth. In order to ensure that the photo-detector is saturated when it is not blocked, the supply current to the LED is usually set to a value that is higher than i_thresh. For example, the supply current on the LED can be set at i_nom, as shown in FIG. 2. Preferably, i_nom is greater than i_thresh, as shown in FIG. 2, however, for testing purposes i_nom may be selected to be the same as i_thresh.
In the past, troubleshooting of photosensors relies on manual testing where the photo-detector is manually blocked and unblocked while the LED is on, and the output voltage of photo-detector is measured using a multi-meter, for example. This manual testing method is cumbersome and sometime impractical. For example, some photosensors in a mailing machine may be located deep inside the machine. Accessing those photosensors may be very difficult.
Thus, it is advantageous and desirable to provide a method and system for testing one or more photosensors in a mailing machine in a systematic and automatic fashion.